Hi Ellen,
The Mitchell Roundhand ones are broad edge, you're right. They can't be used as pointed pens, but they are quite flexible, especially the thinner ones - I am sure they could be used for a form of copperplate. In a different thread (the one about changes in the turn of the letter) there are some examples of English Roundhand that was done with a (narrow) broad nib, and I've been meaning to try it...
I think you'd have better luck sticking to pointed nibs for Copperplate (what we now call English Roundhand) but I always think it's worth experimenting.